The present invention relates generally to magnetic resonance imaging (MRI), and more particularly to a method and apparatus to reduce ghosting artifacts resulting from Maxwell fields and/or other perturbation magnetic fields in MR images acquired using fast imaging techniques.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) which is in the x-y plane and which is at or near the Larmor frequency, the net aligned moment, or xe2x80x9clongitudinal magnetizationxe2x80x9d, Mz, may be rotated, or xe2x80x9ctippedxe2x80x9d, into the x-y plane to produce a net transverse magnetic moment Mt. A signal is emitted by the excited spins after the excitation signal B1 is terminated and this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (Gx Gy and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
It is well known that imperfections in the linear magnetic field gradients (Gx, Gy, and Gz) produce artifacts in the reconstructed images. It is a well known problem, for example, that eddy currents produced by gradient pulses will distort the gradient fields and produce image artifacts. Methods for compensating for such eddy current errors are also well known as disclosed, for example, in U.S. Pat. Nos. 4,698,591; 4,950,994; and 5,226,418. It is also well known that the gradients may not be perfectly uniform over the entire imaging volume, which may lead to image distortion. Methods for compensating this non-uniformity are described, for example, in U.S. Pat. No. 4,591,789.
Other than uncompensated eddy current errors and gradient non-uniformity errors that escape correction, it can be assumed that the magnetic field gradients (Ge, Gy, and Gz) produce linear magnetic fields exactly as programmed, thus spatially encoding the NMR data accurately. With these gradients, the overall static magnetic field at location (x,y,z) is conventionally given as B0+Gx(x)+Gy(y)+Gz(z), and the direction of the field is usually thought to be along the z-axis. This description, however, is not exactly correct. As long as a linear magnetic field gradient is applied, the overall magnetic field direction is changed from the z-axis and its amplitude exhibits higher-order spatial dependencies (x2, y2, z2, z3, . . . ). These phenomena are a direct consequence of the Maxwell equations which require that the overall magnetic field satisfy the following two condition: {overscore (∇)}xc2x7{overscore (B)}=0 and {overscore (∇)}xc3x97{overscore (B)}≈{overscore (0)}. The higher-order magnetic fields, referred to as xe2x80x9cMaxwell termsxe2x80x9d (or Maxwell fields), represent a fundamental physics effect, and are not related to eddy currents or imperfection in hardware design and manufacture.
Many MR scanners still in use to produce medical images require several minutes to acquire the necessary data. The reduction of this scan time is an important consideration, since reduced scan time increases patient throughput, improves patient comfort, improves image quality by reducing motion artifacts and enables dynamic and functional studies. There is a class of pulse sequences which can acquire an image in seconds, or even sub-second, rather than minutes.
One of these fast imaging techniques is the Rapid Acquisition Relaxation Enhanced (RARE) sequence which is described by J. Hennig et al. in an article in Magnetic Resonance in Medicine 3,823-833 (1986) entitled xe2x80x9cRARE Imaging: A Fast Imaging Method for Clinical MR.xe2x80x9d A slight variation of the RARE sequence produces a fast spin echo (FSE) sequence which is used for clinical diagnosis in many commercial scanners. Images acquired using an FSE sequence are very susceptible to artifacts caused by eddy currents induced by the rapidly changing magnetic field gradients. While eddy current compensation techniques are adequate for scans performed with conventional MRI pulse sequences, it has been observed that image artifacts caused by eddy currents are frequently present in FSE scans.
Echo-planar imaging (xe2x80x9cEPIxe2x80x9d) is another ultrafast MR imaging technique which is extremely susceptible to system imperfections, such as eddy currents and gradient group delays. In the presence of eddy currents, ghosting artifacts can considerably degrade the image quality and adversely affect EPI""s diagnostic value.
To minimize the ghosts created by such fast imaging techniques using echo trains, such as FSE and EPI, a common approach is to employ a reference scan prior to the actual image acquisition. In these reference scans, signals from a full echo train are acquired in the absence of the phase-encoding gradient. Each echo in the echo train is Fourier transformed along the readout direction to obtain a set of projections. Spatially constant and linear phase errors, xcfx860 and xcfx861, are then extracted from the projections, followed by phase corrections using xcfx860 and xcfx861, either during image acquisition, as in the case of FSE, or in image reconstruction, as in the case of EPI.
This type of phase correction assumes that spatially varying magnetic fields along the phase-encoding direction are negligible during the reference scans. However, when the Maxwell terms are considered, this assumption does not hold, especially when a strong gradient is used at relatively low main magnetic B0 fields. In the presence of the Maxwell terms, signal dephasing along the phase-encoding direction can introduce substantial errors in the constant and linear phase calculations. The perturbation of Maxwell terms to the reference scans may be evidenced by the fact that the aforementioned phase correction method works markedly well for axial EPI scans performed on a horizontal superconducting magnet, but not for sagittal and coronal scans. In the former case, the EPI readout gradient does not produce Maxwell terms on the phase-encoding axis, whereas in the latter cases substantial Maxwell terms can be produced. In addition to the Maxwell terms, other factors, such as cross-term linear eddy currents from any gradients to the phase-encoding axis, the magnetic field inhomogeneity in the phase-encoding direction, and magnetic hysteresis, can also cause the perturbations to the reference scans, resulting in incomplete or erroneous phase correction.
It would therefore be desireable to have a technique to minimize the effects of the Maxwell terms, as well as other known perturbations, on reference scans using an iterative algorithm to thereby reduce ghosting and other image artifacts.
The present invention relates to a method and system to reduce Maxwell field effects and other perturbation field effects in MR images that overcomes the aforementioned problems.
The invention uses an iterative algorithm in which an image is first reconstructed using extracted constant and linear phase correction values that are obtained in the presence of Maxwell term perturbations. The image is corrected for the Maxwell term induced distortion to yield a new image, which is then used to calculate a phase perturbation error. Once calculated, the phase perturbation error is removed from the projection of the reference scan. With the phase perturbation error being removed from the reference scan, new constant and linear phase correction values are re-calculated from the reference scan, and are used to reconstruct an image with reduced perturbation field effects. The process is repeated until a satisfactory ghost level is achieved.
In accordance with one aspect of the invention, a method to reduce perturbation field effects in MR images includes acquiring an MR image data set having perturbation field effects and acquiring a one-dimensional MR reference scan also having perturbation field effects. The method includes determining phase correction values for the MR reference scan and reconstructing an MR image using the MR image data and the phase correction values. After calculating a projection phase error using the MR image, phase correction on the MR reference scan is performed using the calculated projection phase error to create new reference scan data.
In accordance with another aspect of the invention, a method of reducing ghost artifacts in MR images includes obtaining phase correction values in the presence of Maxwell term perturbations from a reference scan and reconstructing an image using the phase correction values and the acquired MR image k-space data. The method next includes geometrically correcting the image to compensate for Maxwell term induced distortion to create a new image, then calculating a phase perturbation and removing the calculated phase perturbation from the referenced scan.
In accordance with yet another aspect of the invention, an MRI apparatus is disclosed to create MR images acquired with a fast imaging technique that have reduced ghost artifacts. The apparatus includes an MRI system having a plurality of gradient coils positioned about a bore of a magnet to impress a polarizing magnetic field, an RF transceiver system, and an RF switch controlled by a pulse module to transmit RF signals to an RF coil assembly to thereby acquire MR images. The MRI apparatus includes a computer program to acquire an MR reference scan and an MR data set using a fast imaging acquisition pulse sequence where each has perturbation field effects therein. After phase correction values are determined from the MR reference scan, a first MR image is reconstructed using the MR image data set and the phase correction values. A projection phase error is calculated for the first MR image and removed from the MR reference scan to create new reference scan data. A new MR image is reconstructed and displayed which has reduced perturbation field effects. The algorithm is repeated until the displayed MR image has satisfactorily reduced ghost artifacts.
In accordance with yet another aspect of the invention, a computer program is disclosed for use with an MRI apparatus which, when executed by a computer, causes the computer and MRI system to acquire an MR reference scan and MR image data, then obtain phase correction values in the presence of Maxwell term perturbations from the reference scan, reconstruct an image using the phase correction values and the MR image data, geometrically correct the image to compensate for Maxwell term induced distortion and intensity shading to create a new image, and then calculate a phase perturbation and remove the calculated phase perturbation from the MR reference scan. New phase correction values are calculated and a new image is reconstructed using the new phase correction values. The steps of calculating a phase perturbation, removing the calculated phase perturbation from the MR reference scan, calculating new phase correction values, and reconstructing a new image are repeated until a desired ghost artifact reduction is achieved in the final image.
Various other features, objects and advantages of the present invention will be made apparent from the following detailed description and the drawings.